Begin by making the pastry as described in quick flaky pastry. Then leave it to rest in a polythene bag in the refrigerator for 30 minutes.
Meanwhile mix the sausagemeat, onion, sage and seasoning together in a mixing bowl, then divide it into two and roll each piece with your hands on a lightly floured surface to 30cm long.
Put them on a piece of parchment, or a tray, whilst you roll out the pastry.
Preheat the oven to 220C, gas mark 7
Roll out the pastry as described in quick flaky pastry (link) to form an oblong approximately 30 x 20cm. Cut this oblong into two strips 30 x 10cm and divide the sausagemeat also into two, making two long rolls the same length as the strips of pastry (if it’s sticky sprinkle on some flour and flour your hands).
Place one roll of sausagemeat onto one strip of pastry. Brush the beaten egg along one edge, then fold the pastry over and seal it as carefully as possible. Roll the whole thing over so the sealed edge is underneath. Roll lightly and repeat with the second piece of pastry and sausagemeat.
Then use a small sharp knife to cut each roll into six sausage rolls, each about 5cm long.
Snip three V shapes in the top of each roll with the end of some scissors and brush with beaten egg. Place the rolls on the baking sheet and bake high in the oven for 20-25 minutes, then remove them to a wire rack to cool.
You can store the cooked and cooled sausage rolls in an airtight tin, but they do lose their crunchiness. For this reason I think it is preferable to freeze them in polythene boxes and remove a few at a time as and when you need them.
Defrost them for an hour at room temperature and then warm them in a hot oven for 5 minutes.